Both magnetic resonance tomography (MRT) and positron emission tomography (PET) are imaging methods which are suitable for representing the interior of the human body or, especially in the case of PET, for representing biochemical processes in the body. Very recently, there has been interest in combining MRT and PET in one instrument, so that both imaging modes can be employed simultaneously or in quick succession on the same patient.
To this end it has been proposed additionally to provide an optionally removable PET detector inside a magnetic resonance system's conventional magnet arrangement including the base field magnet and gradient coils, optionally as well as a whole-body radiofrequency coil. Such usually replaceable PET detectors are provided in particular for head recordings. The PET detector is enclosed on the outside by a radiofrequency shield. A local coil, in particular a head coil which is also intended to enclose the patient's head, is also provided inside the PET detector during the measurement. This head coil is usually fastened on a patient support and may be operated as a pure transmit coil or as a transmit/receive coil. A patient lying on the patient support, whose head has already been placed inside the head coil, is accordingly moved into the magnetic resonance system and therefore simultaneously into the PET detector ring for the measurement.
The region inside the PET detector, which is moreover designed as a detector ring, should substantially be kept free of materials in order to avoid attenuation and image noise-generating deflection of the PET radiation.
In order to operate the head coil which includes associated electronics, however, it is necessary to connect them to a corresponding connector location arranged on the patient support. Many possibilities are known in respect of this connection, but all of them entail disadvantages.
Some of the terminology used in this application relating to the directions along the longitudinal axis (so-called z axis) of the magnetic resonance system, i.e. the longitudinal axis of both the PET detector and the patient support, will first be established. When a patient is arranged properly on the patient support, their head lies on one side of the patient support and their feet on the other side of the patient support. The direction in which the patient's head points in such an arrangement will be referred to below as “head side”. The opposite direction will be called “foot side”.
In order to connect the head coil to the connector location via a cable connection with at least one releasable attachment device for the electronics and for the connector location, it has been proposed to arrange the coil electronics on the foot side. Although this would avoid additional material being introduced inside the PET detector ring, the electronics are however then arranged over the patient's chest and therefore interfere with their freedom to breathe, so that the patient feels constricted. Fitting the electronics in the region of the patient's neck is likewise found to be inexpedient since recordings are often intended to be made in the cervical vertebra region as well.
Another solution proposes to divide the electronics. Such a head coil or other local coil often consists of a plurality of independently drivable coil elements, to which separate electronics can respectively be assigned. In said case, the electronics may be arranged partially on the posterior side and partially on the chest side. This, however, disadvantageously requires a plurality of cables which are furthermore difficult to access. Especially for heavier patients, exacerbated problems with space are then encountered. Other disadvantages which may be mentioned are large voltage interference through the head coil in the kV range, coupling and distortion of the magnetic fields, susceptibility to vibration and instabilities of the receive coil elements.
For this reason, many head coils are designed so that the electronics are arranged on the head side. The cable connection is then fed through the PET detector and therefore through the radiofrequency field region of the coils. At least one device is, in this case, provided for suppressing the radiofrequency effects, for example surface wave filters, radiofrequency chokes or the like. Nevertheless, the cable connection in the radiofrequency field nevertheless leads to coupling and distortion, so that the symmetry is also broken. The high induced voltages and feedbacks can lead to vibrations and instabilities. Matching furthermore becomes difficult, and decoupling the individual coil elements is complicated. Lastly, in this case it is necessary to arrange additional material inside the annular PET detector, so that attenuation and image noise can occur.